The present invention relates to a nonwoven fabric for pleated filters, partially compressively bonded by embossing and having high weight per unit area, and relates to a production process therefor.
It is generally known that a nonwoven fabric composed of continuous filaments can be produced by drawing the continuous filaments spun from a spinneret by an air sucker, etc. at a high speed, letting them collide with a divergence plate mainly made of a lead, to be electrified and opened, catching them on a net conveyer, and partially thermally compressively bonding them by a pair of heated embossing rolls, to form a nonwoven fabric, or by mechanically entangling the web piled and caught on a net, by needle punching, to form a nonwoven fabric. Such nonwoven fabrics are widely used as industrial materials, geotextiles, etc.
Especially for filters, since the nonwoven fabrics have high performance and are excellent in durability and workability, they are expected to be in great demand as a substitute for the conventional filter paper. For filters, since any partial defect greatly affects the performance of filters, the nonwoven fabric used for them are required to have high quality, being less uneven in weight per unit area and free from partial delamination in the sheet.
Japanese Patent Publication (Kokoku) No. 60-4298 discloses a nonwoven fabric producing process, in which a web composed of continuous filaments of two or more kinds having different melting points is thermally compressively bonded, to melt the filaments of lower melting point, for production of a nonwoven fabric.
However, the nonwoven fabric is insufficient in thermally compressively bonding in the thickness direction even if thermally compressively bonding is effected by a pair of embossing rolls, and delamination is liable to occur at the center of the thickness. Furthermore, since the thermally compressively bonding must be effected at a temperature higher than the melting point of the filaments that have lower melting point, the filaments lower in melting point are liable to adhere to the surfaces of the embossing rolls, and this contamination causes the embossed sheet to be attracted by the rolls, partially delaminating the surfaces of the nonwoven fabric disadvantageously. Therefore, if the nonwoven fabric is later pleated for use as a filter, it cannot be well pleated, and at the partially delaminated portions, fine particles leak. Furthermore, on the partially delaminated surface, a nap is raised, making it difficult to allow dust to be released, lowering the filter performance. Because of these problems, the nonwoven fabric is insufficient as a filter medium.
Japanese Laid-open Patent Application (Kokai) Nos. 2-133644, 2-169756 and 3-8856 disclose nonwoven fabrics composed of sheath-core conjugate filaments respectively consisting of a polymer higher in melting point used as the core component and a polymer lower in melting point used as the sheath component.
The nonwoven fabric disclosed in Japanese Laid-open Patent Application (Kokai) No. 2-133644 has the web thermally compressively bonded on the entire surface, and has smoothness and performance like a film. The weight per unit area specified here is 40 g/m2 at the highest.
Japanese Laid-open Patent Application (Kokai) No. 2-169756 intends to provide a nonwoven fabric having non-melting-bonded portions kept bulky to give a soft fabric feeling, for providing excellent surface abrasion resistance and strength. The weight per unit area specified here is 40 g/m2 at the highest.
Japanese Laid-open Patent Application (Kokai) No. 3-8856 discloses a nonwoven fabric used as a mat in asphalt road construction, waterproof material and soundproof material. The weight per unit area specified here is 100 g/m2 at the highest.
On the other hand, nonwoven fabrics exceeding 100 g/m2 in weight per unit area are disclosed in Japanese Laid-open Patent Application (Kokai) Nos. 3-137261 and 3-146757, but they are impregnated with asphalt or acrylic resin, to be used for roofing.
The object of the present invention is to provide a filter medium having excellent pleatability, freedom from sheet delamination, and high collection efficiency.